@ossobuco,
In gmail, an exchange is threaded (I'm probably not using the right term). That is, there will be something in my inbox that says "John Doe (48)" which means that John Doe and I have exchanged a total of 48 emails on a single topic (he sent one, I replied to that, he replied to my reply, etc.), which are all gathered together. If that thread is deleted, all of the emails are deleted.
That seems to be what the person JPB quoted on the last thread is talking about.
I remember having something like that happen too, when I tried to forward something, maybe.
At any rate, I just don't see (which doesn't mean it's impossible, just that I personally don't see) how a third party would reach into your inbox and get rid of the emails you'd received. A very very tech savvy third party could do it I'm sure, but I'd really suspect a technical glitch over human intention.